1. Field of the Invention
The present invention relates to antennas and wireless communication devices, and more specifically to antennas and wireless communication devices preferably for use in radio frequency identification (RFID) systems.
2. Description of the Related Art
In recent years, RFID systems have been commercialized as article information management systems. In RFID systems, non-contact communication through an electromagnetic field is performed between a reader/writer that generates an induction field and an RFID tag (also called a wireless communication device) attached to an article, whereby predetermined information is transmitted. This RFID tag includes a wireless IC chip that stores predetermined information and processes a predetermined wireless signal and an antenna (radiation body) that transmits and receives a high-frequency signal.
Japanese Unexamined Patent Application Publication No. 2007-96655 discloses an antenna for an RFID tag. In this antenna, which is a bent dipole antenna, impedance is adjusted by forming a slit near a feeding portion. Further, Japanese Unexamined Patent Application Publication No. 2008-160821 discloses an antenna that includes a first loop conductor pattern and second and third conductor patterns connected to the first conductor pattern. In this antenna, which receives circularly polarized waves through the first conductor pattern, impedance is adjusted through adjustment of the lengths of the second and third conductor patterns.
However, the antenna disclosed in Japanese Unexamined Patent Application Publication No. 2007-96655, in which a portion of the dipole antenna is used for impedance adjustment, has a problem in that radiation characteristics such as directivity and gain may be changed depending on a change in the shape of the slit caused by the adjustment. Further, the antenna disclosed in Japanese Unexamined Patent Application Publication No. 2008-160821, in which the first conductor pattern is directly electrically connected to the second and third conductor patterns and, hence, a portion of the first conductor pattern contributes to the impedance adjustment, has a problem in that radiation characteristics such as directivity and gain may be changed as a result of the adjustment, similarly to the antenna disclosed in Japanese Unexamined Patent Application Publication No. 2007-96655.